Modified Project Summary/Abstract Section This project proposes the transition from the K99 to the R00 phase; it is accompanied by the move to a new (R1 level) institution: the Grand Rapids Research Center of Michigan State University, where I have accepted a tenure-track assistant professor position (after NIMH approval of the offer). There are no major changes in the nature of the research proposed versus the K99 phase, only technical differences in the experimental design and expansion of studies already completed. My long-term goal is to inform the etiology of pediatric mental health disorders by determining the normal role of candidate genes in brain development. The near-term goal of this project is to investigate a specific gene: the receptor tyrosine kinase, Kit (herein KitR). Mutations in KitR have been identified in cases of intellectual disability and autism. KitR is abundantly expressed in the brain, but little is known about how KitR may regulate brain development. I propose to address this gap using the mouse cerebellum as a model system due to the high and similar expression of KitR in human versus mouse cerebella and due to the experimental and genetic tractability of mice. Within the cerebellum, Kit Ligand (KitL) is expressed post-synaptically by Purkinje cells (PCs), while KitR is expressed pre-synaptically by Molecular Layer Interneurons (MLIs). I hypothesize that Kit signaling regulates cerebellar development by the mechanism that target-derived KitL attracts KitR expressing neurons to migrate and form synapses. Specific Aim 1 is to test the hypothesis that PC KitL attracts the migration and innervation of MLIs. Specific Aim 2 is to test the hypothesis that MLI KitR is required for the normal innervation of PCs. Specific Aim 3 is to determine by what effectors KitR regulates neuronal migration and synaptogenesis. These Aims will be accomplished by viral and transgenic strategies to knockout, mis-express, or reconstitute endogenous and mutant forms of KitL or KitR in vivo (mouse brain). The impacts on migration, maturation, and connectivity of KitL/R manipulated neurons will then be assayed by microscopy and electrophysiology. These studies will serve NIMH Research Priority 1.1A: to Determine the molecular, cellular, and systems components underlying brain connectivity.